In the case of many objects which are fully or partially formed solidly, their internal structure needs to be examined for material defects. To this end, nondestructive testing methods are required in order to obtain information about the internal structure which cannot be seen. This is necessary in particular for components subjected to heavy mechanical stress.
For example, steel components are forged after casting in order subsequently to be brought into their final shape by turning or other cold treatments. In this case, the testing for internal material defects may be carried out directly after forging.
Such components which are already in use must also be subjected regularly to material testing. This applies in particular to components which are exposed to heavy loads. The component to be tested may, for example, be a turbine blade for a gas or steam turbine. The turbine blade roots, in particular, are exposed to heavy loads during operation. These loads may lead to cracks, which can be detected and located with the ultrasonic measurement method by scanning the surface. Since the surface has a complex geometry, special measurement methods are necessary. Conventionally, such metal parts are tested using ultrasound. In this case, the sound waves which are reflected at interfaces inside the metal part are detected. With the time of flight of the reflected sound wave it is possible to detect, and from this the path length travelled and therefore the distance can be determined. By applying sound from different directions, further information can be obtained about the material defect or defects. From this, material defects can be located. For example, the geometrical orientation of the material defect can be determined in this way. From the shape of the reflected sound waves, deductions can be made about the type of material defect.
By scanning the surface of the test subject using an ultrasonic detector and recording the acquired data, the volume accessible to the ultrasound can be examined fully. From the acquired data, it is possible to generate an image which can be used for assessment.
In one known method, shaped parts are manufactured, for example from casting resin. These shaped parts can be applied with an accurate fit onto the surface to be scanned. The shaped parts contain holes into which an ultrasonic testing head is inserted. In order to be able to scan the entire surface to be tested, the ultrasonic testing head is displaced manually by discrete distances. This, however, is very laborious.
In another known method, the ultrasonic testing head is located on a carriage which is applied by means of a holding device on a neighboring test subject. The holding device can be moved by means of motors over the surface to be scanned. The testing head is pressed onto the surface of the test subject by springs. Optimal orientation of the testing head, however, is not possible in this case.